1. Field of the Invention
The present invention relates generally to apparatus and methods for selective modification and control of a patient's body temperature. More particularly, it relates to relatively non-invasive and field operable systems and methods of lowering a patient's body temperature by heat exchange within the patient's peritoneum. Peritoneal heat exchange provides the benefit of extremely rapid cooling of the patient's target organs such as the heart and brain as well as facilitating global patient body temperature reduction to therapeutically effective temperatures. The heat exchange medium of the present invention is a chilled gaseous fluid suspension of frozen ice particles.
2. Background of the Invention
Man is considered to be a tropical animal. Normal functioning of the human animal requires a body temperature of approximately 37 degrees Celsius (98.6 degrees Fahrenheit). The body can self-compensate for small upward or downward variations in temperature through the activation of a built-in thermoregulatory system, controlled by temperature sensors in the skin. The response to an upward variation in body temperature is the initiation of perspiration, which moves moisture from body tissues to the body surface. When the moisture reaches the surface it evaporates, carrying with it a quantity of heat. The explanation for a person becoming thirsty when exposed to a hot environment for a period of time is that fluids lost due to perspiration must be replaced. The response to a downward variation in body temperature is shivering, which is the body's attempt to generate heat. Shivering is an involuntary contraction and expansion of muscle tissue occurring on a large scale. This muscle action creates heat through friction.
Hypothermia is defined as a core temperature of less than 35 degrees Celsius. Hypothermia is also considered the clinical state of subnormal temperature when the body is unable to generate sufficient heat to effectively maintain functions. Many variables contribute to the development of hypotherthermia. Age, health, nutrition, body size, exhaustion, exposure, duration of exposure, wind, temperature, wetness, medication and intoxicants may decrease heat production, increase heat loss, or interfere with thermostability. The healthy individual's compensatory responses to heat loss via conduction, convection, radiation, evaporation and respiration may be overwhelmed by exposure. Medications may interfere with thermoregulation. Acute or chronic central nervous system processes may decrease the effectiveness of thermoregulation.
Mild Hypothermia is when the core temperature is 34-35 degrees Celsius. The patient is still alert and able to help him/herself and intense shivering begins. The patient's movements, however, become less coordinated and the coldness creates some pain and discomfort.
Moderate hypothermia is when the patient's core temperature is 31-33 degrees Celsius. Shivering slows or stops, muscles begin to stiffen and mental confusion and apathy sets in. Speech becomes slow, vague and slurred, breathing becomes slow and shallow, and drowsiness and strange behavior may occur.
Severe hypothermia is when the core temperature drops below 31 degrees Celsius. Skin is cold, may be bluish-gray in color, eyes may be dilated. The patient is very weak, displays a marked lack of coordination, slurred speech, appears exhausted, may appear to be drunk, denies there is a problem and may resist help. There is a gradual loss of consciousness. There may be little or no apparent breathing, the patient may be very rigid, unconscious, and may appear dead.
Simple methods for treating hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. Even these simple methods may be effective if the hypothermia is not too severe. These simple methods are limited in their effectiveness however. Wrapping the patient in blankets ultimately depends on the patient's own production of heat to rewarm his body. In even moderate cases of hypothermia, or in the case of an ill or injured patient, the patient may simply be too weak or exhausted to produce sufficient heat. Oral administration of a warm fluid requires that the patient be conscious and capable of swallowing the fluid. Since loss of consciousness occurs early in hypothermia, this method is also limited to moderate cases. Immersion of the patient in a warm water bath is often simply impractical. For example, immersion of a patient undergoing surgery would obviously be undesirable. Furthermore, the immersion technique is time consuming and may be ineffective in that it requires the transmission of warmth from the patient's skin surface into the body core before the benefit of the warmth can be realized. Other devices allow for the direct warming of a patient's blood. These methods involve removing blood from the patient, warming the blood in external warming equipment, and delivering the blood back into the patient. While such methods are much more effective than any of the simple methods previously described, they are disadvantageous for other reasons. First, the apparatus involved is quite cumbersome. Second, some danger is involved in even the temporary removal of significant quantities of blood from an already weakened patient. In fact, a further drop in body temperature is often experienced when blood is first removed for warming in the external apparatus. Finally, special catheters are used for the direct warming of a patient's blood. However, those catheters require a trained staff to insert the device to a central blood vessel of the patient and those physicians are available only in specific units and not in the ambulance or even not always in the emergency room. Those instruments are also very expensive and thus are not available for every caregiver.
Recent medical reports have described the use of controlled hypothermia as a means to reduce oxygen consumption of tissue, such as the heart muscle and the brain during decreased perfusion that occurs as a result of myocardial infarction and ischemic stroke (respectively), which leads to reduced damage and decrease of the infarcted area. Medical reports have also described the prophylactic use of controlled hypothermia during cardiac surgery or interventional cardiology procedures for reducing damage from ischemia and/or embolization in the heart and brain during and after the procedure.
The ability to prevent or greatly reduce long term damage to cardiac or brain tissue while treating patients for myocardial infarction and stroke provides a compelling need for methods and systems for purposefully inducing therapeutic hypothermia in controlled effective manner. Such systems are ideally portable and deployable by emergency medical responders in the field and must be capable of rapidly cooling vital heart and brain tissues to prevent as much damage as possible. As of yet an ideal system or method for rapidly inducing hypothermia non-invasively and outside of a critical care hospital setting does not exist. Cooling blankets offer a portable easily deployable means of chilling a patient but the body's own thermoregulatory mechanisms counteract the cooling mechanisms of such blankets through vasoconstriction. As a result cooling blankets are not able to induce hypothermia in the patient in clinically relevant time span. Ice baths are capable of reducing patient body temperature rapidly due to the large thermal gradient and large specific heat capacity of the cooling medium. However ice baths are not portable are inconsistent with necessary concurrent interventions required for treatment of MI and stroke, such as balloon angioplasty. Peritoneal catheters equipped with heat exchangers are capable of rapid cooling of the patient but the size required of such catheters makes their deployment invasive. Additionally such catheterizations require skilled technicians and must be performed in the hospital. By the time a patient has reached a hospital much critical time has been lost. Field deployable respiratory cooling systems that operate by using the body's own lungs as heat exchangers and use a gaseous fluid suspension of frozen particles as a convective cooling medium are capable of inducing hypothermia in clinically relevant time spans. However, often in medical emergencies such as stroke or MI the patient exhibits poor or depressed respiration. Additionally, respiratory cooling mechanisms have yet to match cooling rates of peritoneal cooling.
The following patents and patent applications describe apparatus and methods for affecting a patient's body temperature. These, and all other patents and patent applications referred to herein, are hereby incorporated by reference in their entirety.
3. Background Art
U.S. Pat. No. 8,100,123 and US2012/0167878 commonly assigned with the present application describe method and systems for delivering a frozen mist in a breathing gas to a patient to achieve hypothermia. The full disclosures of these patent documents are incorporated herein by reference.
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